The invention relates to an active matrix liquid crystal display (AMD) being operated in the second or a higher transmission minimum of the Gooch-Tarry curve and to stable nematic liquid-crystal compositions with high optical anisotropy for use in such AMD""s, e.g.)for projection systems.
Active matrix displays (AMD) are highly favored for commercially interesting displays with a high information content. Such AMDs are used for TV application and also for displays for e.g., laptops, automobiles and aeroplanes.
AMDs have non-linear electrical switching elements which are integrated at each picture element. As non-linear driving elements thin film transistors (TFT) [Okubo, U., et al., 1982, SID 82 Digest, pp. 40-41] or diodes (e.g.: metal insulator metal: MIM) [Niwa, K., et al., 1984, SID 84 Digest, pp. 304-307] can be applied. These non-linear driving elements allow to use an electro-optical effect with a rather flat electro-optical characteristic if a good viewing angle characteristic can be obtained. So a TN-type LC cell [Schadt, M. and Helfrich, W., 1971, Appl. Phys. Lett., 18, 127] with a twist angle in the region of 90xc2x0 can be used. To provide the good contrast over a wide viewing angle, operation in the region of the first minimum of transmission [Pohl, L., Eidenschink, R., Pino, F., del. and Weber, G., 1980, German Pat., DBP 30 22 818, and 1981, U.S. Pat. No. 4 398 803; Pohl, L., Weber, G., Eidenschink, R., Baur, G., and Fehrenbach W., 1981, Appl. Phys. Lett., 38, 497; Weber, G., Finkenzeller, U., Geelhaar, T., Plach, H. J., Rieger, B., and Pohl, L., 1988, Int. Symp. on Liq. Cryst., Freiburg, to be published in Liq. Crys.] is favored. These AMDs are very well suited for direct view and projection type TV-displays and consequently are of high commercial interest. For these applications some physical properties of the liquid crystals become more important than for passive TN displays. Some of the decisive properties for the performance of an AMD are resistivity and UV- and thermal stability of the liquid crystal [Togashi, S., Sekiguchi, K., Tanabe, H., Yamamoto, E., Sorimachi, K. Tajima, E., Watanabe, H., Shimuzu, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; Stromer, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. A problem often encountered is the adverse influence of UV-illumination on the resistivity and therefore on the general performance of the liquid crystal mixture in the display.
In an AMD the non-linear switching elements are addressed in a multiplex scheme. So they charge the electrodes of a pixel in the limited time they are active. Then they become inactive until they are addressed again in the next cycle. Consequently the change of the voltage on an activated (charged) pixel is a nondesired but a very decisive feature of such a display. The discharge of a pixel is determined by two factors. These are the capacity of the pixel element including liquid crystal and the resistivity of the dielectric material between the electrodes, namely the liquid crystal. The characteristic time constant of the decay of the voltage at a pixel (RC-time) has to be significantly bigger than the time between two addressing cycles (tadr.). A parameter frequently used to describe the performance of an AMD is the voltage holding ratio HR of a picture element:   HR  =                    V        ⁡                  (          to          )                    +              V        ⁡                  (                      to            +                          tadr              .                                )                            2      ⁢              xe2x80x83            ⁢              V        ⁡                  (          to          )                    
As the voltage at a pixel decays exponentially an increase of the holding ratio necessitates liquid crystal materials with exceptionally high resistivities.
There are several points of importance for the resistivity of the liquid crystal inside a display, e.g., orientation layers, curing condition of the orientation material. But by no means less important are the electrical properties of the liquid crystal used. Especially the resistivity of the liquid crystal in the display determines the magnitude of the voltage drop at the pixel.
Earlier investigations with low-xcex94n materials have shown, that the requirements with regard to resistivity and UV-stability and temperature dependence of the resistivity for TFT-applications cannot be met with materials containing cyano moieties as terminal groups. Non-cyano materials containing halogenated terminal groups can show far better resistivity values and UV-stability as well as superior viscosity values than conventionally used cyano materials. However, in general these non-cyano materials unfortunately show a strong tendency towards forming smectic phases, especially at low temperatures, the clearing points and the dielectric anisotropy values of non-cyano materials with halogenated terminal groups are much lower.
Modern commercial mixtures have to operate over a wide temperature range; therefore, crystallization or formation of smectic phases at low temperatures has to be excluded. Good solubility is one of the most important preconditions for the usability of liquid crystalline materials in the development of nematic mixtures. Compounds with high melting temperatures or a tendency to form smectic phases are for this reason not suitable.
By very careful selection of the components and an appropriate mixture design it was possible to find low birefringence non-cyano mixtures having a broad nematic temperature range for first minimum application [B. Rieger et al., Proc. 18. Freiburger Arbeitstagung Flussigkristalle, Freiburg 1989, 16 (1989)]. Non-cyano materials with high birefringence, which are essential for the mixture concept of this invention unfortunately show in many cases even more unfavorable properties such as high melting points and/or strongly smectogenic behavior than similar materials with lower birefringence:
Mixtures of the state of the art with a birefringence suited for operation in the second or a higher transmission minimum of the Gooch-Tarry curve are not acceptable for active matrix application.
There is thus still a great need for liquid-crystal composition having a high resistivity and other suitable material properties for use in As.
The invention has for one of its objectives to provide a nematic liquid crystal mixture having a positive dielectric anisotropy xcex94∈ of at least +4 and a birefringence xcex94n of at least 0.12, characterized in that the mixtures comprises one or more components having the core structure 
wherein L1, L2, Y and Z are each independently of one another H or F, one of Q1 and Q2 is 1,4-phenylene, 3-fluoro-1,4-phenylene or 3,5-difluoro-1,4-phenylene and the other residue Q1 or Q2 is xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2CH2CH2xe2x80x94 orxe2x80x94if at least one of L1, L2, Y and Z denotes F xe2x80x94also a single bond, whereby this core structure can be optionally further fluorinated in the benzene rings.
The invention has also for its objective to provide an matrix liquid crystal display with high temperature and UV-stability containing
two plane parallel support plates which together with a frame form a cell of the thickness d,
integrated non-linear elements for switching individual picture elements on the support plates, and
a nematic liquid crystal mixture which is present in the cell, has a positive dielectric anisotropy and a birefringence xcex94n,
the display being operated in the second or a higher transmission minimum of the Gooch-Tarry curve by appropriate selection of d. xcex94n, characterized in that the quotient of the voltage holding ratio HR20 after 20 hours exposure to UV-light (280-400 nm, 12 mW/cm2) and HRo before exposure to UV-light is larger or equal to 98% and also liquid crystal compositions with a very high resistivity which meet also the other demands.
It has now been found that such values for the HR are even possible for mixtures with higher birefringence by using laterally fluorinated and/or ethyl-linked non-cyano materials. Very high RC time values can be obtained in AMDs. These mixtures also show a reduced viscosity and allow short switching times at reasonable threshold voltages.
The thickness of the AMDs is preferably in the range of 3 to 10 xcexcm. Especially preferred is the range from 3 to 7 xcexcm.
The following preferred embodiment concern the nematic liquid crystal mixture which is present in the AMD:
The birefringence xcex94n of the nematic liquid crystal mixture is 0.12 to 0.20, preferred 0.13 to 0.18.
The dielectric anisotropy of the nematic liquid crystal mixture is at least +4,0.
The liquid crystal mixture contains one or more compounds of the formula I 
wherein R is alkyl or alkoxy of up to 10 carbon atoms
r is 0 or 1,
X is F, Cl or a fluorinated and/or chlorinated alkyl, alkenyl or alkoxy group of 1, 2 or more carbon atoms, and
L1, L2, Q1, Q2,
Y and Z have the meaning given above.
The liquid crystal mixture contains one or more compounds of the formula II 
wherein R is alkyl or alkoxy of up to 10 carbon atoms
r is 0 or 1,
X is F, Cl or a fluorinated and/or chlorinated alkyl, alkenyl or alkoxy group of 1, 2 or more carbon atoms, and
L1, L2, A is trans-1,4-cyclohexylene or 1,4-phenylene,
Y and Z have the meaning given above,
with the proviso that at least one of L1, L2, Y and Z is F.
The liquid crystal mixture contains one or more compounds of the group consisting of III to IX 
wherein n is preferably 1 to 7 and X denotes F, Cl, CF3, OCF3 or OCHF2.
The compounds shown above are known from, e.g., DOS 30 42 391, DOS 39 02 328, DOS 39 13 554, DOS 39 09 802, WO 89/02884, WO 90/15113, WO 90/09420, the International Patent Appln. No. PCT/EP 90/01292, No. PCT/EP 91/00411, No. PCT/EP 90/01471, No. PCT/EP 90/0 2109 and the European Patent Appln. No. 9 1 100 675.7 or can be prepared in analogy to known compounds.
The mixtures according to the present invention usually are based on the medium polar components having the indicated core structure and other non-cyano components. Of course, however, such mixtures can also additionally contain known cyano LC components if extremely high values for the HR are not needed, e.g., for TN or STN-use. Such mixtures can also contain tolan components for adjusting extremely high xcex94n values. The resulting mixtures are important for achieving very broad nematic phase ranges including very low temperatures (outdoor use).
The highly advantageous properties of the claimed mixtures are achieved by using components having the core structure: 
The nature of the remaining terminal groups is not very critical an there can be used successfully a vast multitude of non polar and medium polar halogenated terminal groups. The terminal group located at the X and Y substituted ring is a medium polar group such as F, Cl or a fluorinated and/or chlorinated alkyl, alkoxy or alkenyl group of 1, 2 or more carbon atoms. Preferred groups are F, Cl, CF3, OCF3, CHF2, OCHF2, OCF2Cl, OCF2CF2H.
The other terminal group is preferably a non polar group such as R, 
R is preferably an alkyl, alkoxy, oxaalkyl, dioxaalkyl, alkenyl, fluoroalkyl or chloroalkyl group of in each case up to 10 carbon atoms.
The preparation of the mixtures according to the invention is effected in the conventional manner. In general, the desired amount of the components which is used in the smaller amount is dissolved in the components which constitutes the main constituent, preferably at elevated temperature. If this temperature is chosen to be above the clear point of the main constituent, the completeness of the process of dissolving can be observed particularly easily.
However, it is also possible to mix solutions of the components in a suitable organic solvent, for example, acetone, chloroform or methanol, and to remove the solvent without introducing any contaminants or undesirable dopants.
By means of suitable additives the liquid crystal phases according to the invention can be modified in such a way that they can be used in any hitherto disclosed kind of AMD.
Especially preferred are the mixtures of the present invention for use in active matrix projection systems including PDLC type systems.